1 



TERRESTRIAL MAGNETISM. 



[S<ict. II. 



was supposed to he its minimum was inferred ; and from 

 the result so ohtained^ combined with a similar compari- 

 son made by myself between Paris and London in 1827 

 with several magnets, the ratio of the force in London to 



M. do H 



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and thus to afford a secure unvarvinsj basis foi 



been inferred to be 1'372 to 1-000. This is the 

 origin of tlie num^ber l'372j which has been generally em- 

 ployed by British observers, not furnished with the means 

 of making absolute determinations, to express the vnlue 

 of the magnetic force at their base station, viz.j London. 

 The essential disadvantage, ho. /ever, under which any re- 

 lative scale of the nature referred to labours, is, that the 

 magnetic force of the Earth has been found to be subject 

 to secular variations, so that at no one spot on the surface 

 of the globe can the intensity be assumed to remain con- 

 such 



a scale ; whereas by absolute measurements, we are not 

 oxdy enabled to compare numerically with one another the 

 results of experim.ents made in the most distant parts of 

 the globe, with apparatus not previously compared, but 

 we also furnish the means of comparing hereafter the in- 

 tensity which exists at the pre^^ent epoch, with that which 

 may be found at future periods. It is probable from these 

 and other considerations, that the employment of mere 

 relative scales will shortly be entirely superseded by the 

 general adoption of a scale in which the value of the force 

 is expressed in terms of a fixed and unchanging unit. 



6. The instrument with which the absolute value of the 

 horizontal component of the force is measured is called the 

 Unifilar Magnetometer ; its description, and that of the 

 process by which results are obtained with it, are given in 



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